His sin this time was to criticize fellow Democrats for demonizing the private equity industry, in which Mitt Romney made his fortune at Bain Capital. The Obama campaign recently ran an attack ad over Bain’s purchase of GS Technologies, a steel firm in Missouri that eventually went bankrupt, costing 750 jobs.

“It was like a vampire,” one steelworkers says in the ad. “They came in and sucked the life out of us.”

You get the idea: Romney is the rich guy counting his gold coins, while the blood of the middle class drips from his fangs.

The ad doesn’t mention that Bain invested tens of millions of dollars trying to save the steel mill, that cheap imported steel killed other mills during this period, or that Romney was not running Bain at the time of the bankruptcy. He had moved on to oversee the Olympic Games in Salt Lake City.

Private equity firms are just trying to make money. Sometimes, they invest in promising businesses and create jobs. Sometimes, they make bad bets and kill jobs. But allowing capital to move to its most productive spot creates more wealth on the whole. It is nuts to suggest that is immoral.

Yes, you can challenge Romney over Bain in a fair way. It’s not clear that Bain created any net new jobs for the economy, as Romney claims. And Democrats may find evidence of parasitic behavior at some point. But GS Technologies isn’t it.

Booker raises lots of money from Wall Street, so the hard left will attack his motives. Give him this, though: In a hyperpartisan age, he is charting his own course. He backed pension and health reforms to protect his city from bankruptcy, while most Democrats in Trenton rushed to obey the unions. Another example: He supports vouchers as a last resort for kids stuck in failing schools.

To us, his willingness to defy the high priests of his party is nothing but refreshing.